The Mutator system is most mutagenic of the known plant transposable element systems and has been successfully harnessed to tag numerous maize genes. A member of the MuDR class of elements regulates the Mutator system. MuDR encodes two genes, mudrA and mudrB. Both mudrA and mudrB are required for completion of the transposition process. Previous studies have identified roles for MURA protein, while the role(s) of MURB protein remains undefined. Analysis of elements already sequenced in Arabidopsis and rices shows that in these species MuDR elements seem to lack mudrB, suggesting that mudrB may have been acquired in only a subset of plants, including maize. The ultimate goal of this proposal is to understand the role mudrB plays in the activity of Mutator transposons in maize. To achieve this goal we will examine the distribution and sequence characteristics of mudrB-like sequence isolated from a variety of grass species. Our working hypothesis, based on preliminary analysis of mudrB copy number and distribution in various grasses is that mudrB is a "captured" host sequence in maize. In addition, the role(s) of MURB will be examined using available MuDR deletion derivatives and a series of plant transformations. The essential role of MURB coupled with the importance of the Mutator system as a tool for studying maize gene expression and function, necessitates its understanding.